The invention relates to a system and method for manufacture of an optical fiber preform and optical fiber, and more particularly to a method and apparatus for preparing a preform""s tip to expedite fiber draw.
The starting point in the method for manufacturing optical fiber is the preparation of an elongated, cylindrical quartz glass body, doped accordingly to provide the desired refractive index profile in the optical fiber. This so-called xe2x80x9cpreformxe2x80x9d is introduced very slowly, by means of a downfeed device, into the high-temperature heating furnace of a draw tower. The preform is heated at its terminal end to temperatures sufficiently high to cause the tip of the preform to be gradually transformed to a honey-like consistency. After reaching the appropriate temperature, a glass gob melts off the preform by the influence of heat and gravity. Then an operator hand strips additional trash glass from the preform until a portion of the preform being drawn from contains good quality glass. At the point where good quality glass is readily strippable, the so-called xe2x80x9cdraw tipxe2x80x9d has been formed on the preform. The draw tip consists of a generally tapering transition area located between the quartz glass cylinder portion of the preform and the thin fiber strand being drawn from the lowermost tip of the preform. The dimensions of the drawn fiber depend on the high-temperature furnace and the individual drawing conditions. However, a preferred glass strand of approximately 125 xcexcm in diameter is continuously drawn off from the preform. It then passes through the cooling tube and passes through the coating system, which applies the protective sheath (coating). The coating system is located in the drawing tower below a diameter testing device. At the end of the draw tower, the fiber is wound onto a spool.
After a significant length of optical fiber is drawn, the length being dependent on the size of the preform, the preform is consumed. Production must be interrupted and the consumed preform must be replaced by a new one. Frequently, more than one hour elapses until the production of fiber with the required characteristics resumes. During this start-up time, the gobbing off of the quartz glass mass that cannot be used takes place as well as stripping and shaping of the draw tip. The shaping of the draw tip alone takes approximately 30 to 40 minutes. This is valuable time spent on the draw apparatus that, in some cases, is the bottleneck in the production process.
According to the invention, a system and method are provided. In particular a pregobbing system and method are provided for the pregobbing treatment of an optical fiber preform to provide a desired pre-optimized tip geometry prior to insertion into a draw apparatus. The method and system provide a preform having an optimized shape draw tip and permits, for example, the glass mass of an optical fiber preform that is not suitable for fiber drawing to melt off very rapidly and efficiently. The pregobbing operation is performed in a pregobbing apparatus separate from the draw furnace of the draw tower. Thus, since the pregobbing is accomplished in an xe2x80x9coff linexe2x80x9d fashion, i.e., in another apparatus, the fiber throughput from the draw apparatus is increased. Thus, it should be recognized that preforms including pre-optimized leading end tips (which are already appropriately melted off) result in considerably shortened downtime for the optical fiber draw apparatus.
In accordance with an embodiment of the invention, a system is provided for manufacturing an optical fiber preform comprising a pregobbing furnace adapted to heat the preform and cause glass to be removed, the pregobbing furnace having a temperature profile that is substantially the same as a temperature profile of a draw furnace utilized in a subsequent process to draw fiber from the preform. By having substantially equal temperature profiles, the tip is of the exact shape it would have been had the tip preparation been performed in the draw apparatus.
In accordance with another embodiment of the invention, a system is provided for manufacturing an optical fiber preform. The system comprises a pregobbing apparatus having a heating furnace with a first temperature profile, the pregobbing apparatus adapted to provide a pre-optimized tip shape on the optical fiber preform, and a draw furnace having a second temperature profile which is substantially equal to the first temperature profile, the draw furnace adapted to draw fiber from the preform having the pre-optimized tip shape. Preferably, the pregobbing apparatus includes an induction heater. Most preferably, both the pregobbing and the draw apparatus each include an induction heater.
In accordance with another embodiment of the invention, a method for manufacturing an optical fiber preform is provided comprising heating a consolidated optical fiber preform with an induction heating apparatus having a first temperature profile to allow a gob to drop under the influence of heat and gravity, removing glass from the preform until a draw tip having a pre-optimized shape is formed, and transferring the preform to a draw furnace of a draw apparatus. Preferably, the pre-optimized shape includes a tip taper having a tip length to change in radius ratio of between about 5 to about 12, and most preferably between about 6 to about 9. The xe2x80x9cpre-optimizedxe2x80x9d tip shape as the term is used herein means a tip that has been pre-shaped such that it has a taper approximately equal to what it will have when fiber is being drawn therefrom in the draw apparatus. The ratios mentioned above represent a pre-optimized tip taper in accordance with the invention.
In accordance with another embodiment of the invention, a method for manufacturing an optical fiber is provided comprising the steps of heating a plurality of consolidated optical fiber preforms within a plurality of pregobbing apparatus, each apparatus including a furnace having a first temperature profile to form pre-optimized shape preform tip on each of the plurality of preforms, and transferring the plurality of preforms to a plurality of draw apparatus, each draw apparatus including a furnace and drawing optical fiber therefrom, the plurality of draw furnaces each having a second temperature profile substantially equal to the first temperature profile. Accordingly, a large number of draw apparatus may be supplied with pre-optimized preforms by a small number of pregobbing apparatus.
From the foregoing, it should be apparent that by using appropriately pregobbed preforms, that is, preforms that have pre-optimized draw tip shape, it is possible to advantageously reduce the down time of a glass fiber drawing unit by up to 60 minutes per replaced preform. In particular, since the high temperature draw furnace of the drawing tower needs to melt less quartz glass mass, this results in shorter heat-up times. Further, the transition phase between the initial melting and fiber draw takes considerably less time. Thus, the draw tower is in operation and producing production quality fiber more of the time. This is particularly true when the draw tip is optimized off-line in a separate pregobbing apparatus. In addition, it is not necessary to move the preform as deep into the draw furnace (time savings because of the generally low feed rate). Furthermore, stripping time at the draw tower for stripping off inadequate quality fiber is reduced. Thus, it should be recognized that off-line pregobbing represents a significant time savings.